It is well known that optical fibers can be coated with protective organic coatings as the glass fibers are drawn from a glass melt or solid glass preform. Glass fibers, as drawn, exhibit very high tensile strength, but are substantially weakened by the development of flaws on the surface of the fiber. Thus, application of a protective coating to the surface of the fiber before it contacts any surface contaminants or solid surface can protect the inherent high strength of the fiber.
A number of different coating systems have been used commercially in the production of optical fibers for optical telecommunications. However, only two such systems are presently in widespread commercial use.
One system employs coating materials which are rapidly cured by exposure to ultraviolet light. Examples of such coatings are UV-curable urethane acrylates, such as those described in European Patent No. EP0204160. Ultraviolet-curable acrylate coating systems provide acceptable service over a relatively broad range of ambient temperatures. They are not, however, sufficiently stable to withstand elevated temperatures for prolonged periods of use.
Glass optical fibers have also utilized silicone polymer coating systems. Examples of such optical fibers include U.S. Pat. Nos. 4,765,713 to Matsuo et al., 4,848,869 to Urruti, 4,877,306 to Kar, and 4,962,996 to Cuellar et al. Generally, these products are prepared by coating optical fibers with silicone resin and then curing the coated fibers, usually at elevated temperatures and pressures. Curing causes a cross-linking reaction between --Si--CH.dbd.CH.sub.2 and --Si--H groups to form --Si--CH.sub.2 --CH.sub.2 --Si-- cross-links. This reaction is usually catalyzed by a hydrosilylation catalyst incorporated in the silicone polymer coating. See e.g., U.S. Pat. No. 4,689,248 to Traver et al.
One major problem encountered in optical fibers is the absorption of gaseous hydrogen. This gas can attenuate signals transmitted at wavelengths greater than 1 micron which are the wavelengths conventionally used in telecommunications. Hydrogen can also degrade the mechanical characteristics of optical fibers.
Hydrogen which contacts optical fibers can come from outside the cable by diffusion through the cable components. In addition, hydrogen is often generated by materials forming the cable which either have absorbed hydrogen during manufacturing or have decomposed during use. For example, hydrogen can form in metallic or plastic sheaths in plastic cores, in metallic armors, in silicone coatings, and in protective means for the optical fibers, (e.g., tubes loosely housing the optical fibers).
One particularly significant source of hydrogen in optical fiber cables are --Si--H groups present in the silicone coating. As discussed above, the objective of the curing step is to react such groups with --Si--CH.dbd.CH.sub.2 groups to effect cross-linking. However, as cross-linking proceeds, the silicone polymer chains become locked into position, causing some --Si--H groups to become isolated from --Si--CH.dbd.CH.sub.2 groups which they must react with. As a result, --Si--H groups remain in the silicone coating after curing is completed. Over time, such unreacted --Si--H groups generate deleterious quantities of hydrogen by what is believed to be the following catalyzed hydrolytic reaction: EQU --Si--H+H.sub.2 O.fwdarw.--Si--OH+H.sub.2
In addition, hydrogen can be further produced by the following reaction when --Si--OH groups are already formed. EQU --Si--OH+--Si--H.fwdarw.--Si--0--Si--+H.sub.2.
In commercial operations, the residual --Si--H groups remain in cured silicone coatings at levels which will produce 2000-3000 microliters of hydrogen per gram of silicone coating (measured with a gas chromatograph). Such levels of hydrogen are likely to cause the above-described signal attenuation and mechanical degradation problems. As a result, removal of residual --Si--H groups is highly desirable.
One approach to the hydrogen generation problem is to incorporate a hydrogen fixing filler in the optical cables, as disclosed by U.S. Pat. No. 4,688,889 to Pasini et al. Though such techniques may be effective, they significantly increase the weight and material costs of the resulting optical fiber product.
The more preferred approach is to prevent hydrogen from ever being formed by substantially eliminating --Si--H groups. Achievement of this has been attempted by increasing curing times and temperatures.
When curing time is increased, however, a mode unsuitable for commercial operations, residual --Si--H groups will still be present at levels sufficient to generate 500 microliters of hydrogen per gram of silicone coating. Such hydrogen levels are still too high, and such increased curing times necessitate use of longer, more expensive curing ovens.
If it is instead attempted to increase curing temperature to a level which substantially eliminates --Si--H groups, the silicone coating may burn or have its physical properties altered. This undesirable effect can only be prevented by use of a special low oxygen content atmosphere in the curing oven such as that taught by U.S. Pat. No. 4,679,899 to Kobayashi et al. The use of this technique is, however, undesirable due to the need for special gas handling systems.
As a result, the need for an economical procedure to reduce --Si--H groups in cured silicone-coated optical fibers continues to exist.